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258

ALLOYS OF LEAD AND TIN

ALLOYS   OF  LEAD   AND   TIN

The lead-tin alloys are used for soldering and for many other purposes,
such as making ornamental articles, statuettes, toys, tubes for paints and
perfumery, tin-foil, etc.

In these alloys, the essential determinations are those of the lead and
tin (see i). Sometimes, however, for special products, the determination
of the lead alone or of the tin alone is of interest (see 2), whilst in other
cases, determination of the impurities (Cu, Sb, As, etc.) is desired (see White
Metals).

1. Determination of the Lead and Tin.—About i gram of the alloy
as filings is treated in a covered beaker with 6 c.c. of fuming nitric acid
(D = 1-5) and 3 c.c. of water then poured carefully down the side of the
beaker so that the two liquids mix slowly and the action proceeds slowly
and regularly. When the action is complete, the liquid is heated to boiling,
diluted with 50 c.c. of boiling water, and, when the liquid above the pre-
cipitated metastannic acid has become clear, filtered by decantation through
a close filter-paper. The precipitate is washed by decantation with water
slightly acidified with nitric acid, collected on the filter, washed with hot
water x and treated as in B. The filtrate is utilised as in A.

A.  DETERMINATION OF THE LEAD.

1.  ElectrolyticMy.   The filtrate is made up to volume and an aliquot
part, containing about 0-2-0-25 gram of lead, is treated with 1-2 grams
of pure copper dissolved in 10-15 c.c. of nitric acid, diluted to 200-250 c.c.,
and electrolysed at the ordinary temperature : Winkler anode ; cylindrical
gauze cathode; ND100 = 0-3 amp. ; voltage = 2-2-2 ; and duration,} 10-15
hours.

When by raising the level of the liquid the deposition of the lead is found
to be complete—even although copper may still remain in solution—the
electrolytic beaker is replaced by another filled with distilled water, the
current being maintained. After some time the anode coated with lead
peroxide is detached, washed with water alone and dried at 200-220°. The
factor for converting weight of peroxide into that of lead is 0-866, 0-865
or 0-8635 according as the amount of the peroxide is less than o-i gram,
between o-i and 0-3 gram, or above 0-3 gram (Classen).

2.  Grammetrically.   The filtrate is evaporated in presence of 3-4 c.c.
of cone, sulphuric acid until white fumes appear.    When cold, the liquid
is taken up with 50 c.c. of water, heated gently and stirred, allowed to cool
and treated with 15 c.c. of 95% alcohol.   After 1-2 hours, the lead sulphate
is collected in a Gooch crucible and weighed (see Gravimetric Determination
of Lead in Ordinary Brass).

B,  DETERMINATION OF THE TIN.

i. Ekctrolytically.   The metastannic acid precipitate with the filter-

» When fuming nitric acid is used, the metastannic acid separating is practically
free from lead (Busse : Zeitschr. analyt. Chem., 1878, XVII, p. 53). If it is desired to
determine the minimal quantities of lead retained in the metastannic acid, the methods
indicated for the determination of tin in ordinary bronzes (see p. 232) may be used.subsequent tinning, the